Pinus contorta Dougl. (lodgepole pine) is the most important seral tree species in the Canadian Rocky Mountains, forming a complex mosaic of even— and several—aged forests of fire origin over a wide range of elevations and habitats. East of the Continental Divide in Alberta it is the most abundant tree of montane and lower subalpine forests, due to extensive and frequent wildfires before 1910. Sixty—three mature, undisturbed P. contorta forest stands were surveyed for physiognomy, vascular floristic composition, species cover, age structure, and physical site properties. A direct gradient ordination of stands was constructed using elevation corrected to 53°N as the first axis, and Synthetic Moisture Index derived from Moisture Indicator Values of species as the second axis. Stands were classified using minimum—variance cluster analysis with species cover data, error—sum—of—squares association analysis with species presence—absence data, and dominance analysis with species based on the leading species of shrub and herb—dwarf shrub strata. Vascular species richness per stand averaged 38. Species occurring in a significantly higher percentage of stands in one national park than the other were more numerous in Banff than Jasper. Most stands were<100 yr old. Prevailing soils were Eluviated Eutric Brunisols and Orthic Eutric Brunisols. Both vascular species and community classification (syntaxonomic) units showed ecologically meaningful distribution and abundance patterns on the ordination. Though based on different criteria, the four classifications were quite similar, permitting synthesis of five PINUS CONTORTA community types (CT's), these forming an ecocline from low—elevation dry to high—elevation moist: PINUS CONTORTA/SHEPHERDIA CANADENSIS/ARCTOSTAPHY—LOS UVA—URSI (PICO/SCHA/ARUV), P. CONTORTA/S.CANADENSIS/ELYMUS INNOVATUS (PICO/SHCA/ELIN), P. CONTORTA/ALNUS CRISPA/LINNAEA BOREALIS (PICO/ALCR/LIBO), P. CONTORTA/MENZIESIA GLABEL—LA/VACCINIUM SCOPARIUM (PICO/MEGL/VASC), P. CONTORTA/LEDUM GROENLANDICUM/V. SCOPARIUM (PICO/LEGR/VASC). Stands of PICO/SHCA/ARUV are mostly seral to Pseudotsuga menziesii, of PICO/SHCA/ELIN to either Picea glauca or Pseudotsuga menziesii, of PICO/ALCR/LIBO to Picea glauca—Abies lasiocarpa, of PICO/MEGL/VASC to Picea engelmannii—Abies lasiocarpa, and of PICO/LEGR/VASC mostly to Picea mariana. Pinus contorta may be climax on dry, oligotrophic sites above the cold elevational limit of Pseudotsuga. A stable species structure (dominance hierarchy) develops in subordinate vascular strata of seral Pinus contorta forests long before the pine is replaced by climax tree species. The P. contorta forests of Banff and Jasper are compared with those of adjoining regions in Alberta, British Columbia, and Montana.